1. Field of the Invention
The present invention relates to a laser welding method employing a CO2 laser generator to generate the welding laser beam and a protective gas free of helium or containing only a low proportion of helium.
2. Related Art
Laser beam welding is a highly efficient joining method because it serves to obtain very high penetration depths at high speeds, in comparison with other more conventional processes, such as plasma welding, MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.
This is because of the high power densities employed when one or more mirrors or lenses are used to focus the laser beam on the junction plane of the parts to be welded, for example power densities up to more than 106 W/cm2.
These high power densities cause a strong metal vaporization at the surface of the parts to be welded which, by expanding outwardly, causes a progressive hollowing of the metal pool and leads to the formation of a narrow and deep vapor capillary called “keyhole” in the thickness of the parts, that is, at the junction plane. Said capillary allows the direct deposition of the laser beam energy deep in the thickness of the parts, as opposed to more conventional arc welding processes, in which the energy deposition takes place at the surface.
The vapor capillary consists of a mixture of metal vapors and metal vapor plasma, which has the property of absorbing the laser beam and therefore trapping the energy in the capillary itself.
The propagation of the metal plasma outside the capillary generates a hot and radiant metal plasma plume.
A known problem in laser welding employing a CO2 laser device generating a laser beam with a wavelength of 10.6 μm is the formation of an undesirable plasma in the blanket gas or protective gas.
This is because, by seeding the blanket gas or protective gas with free electrons, the metal vapor plasma can initiate an ionization therein. The ionization of the blanket gas can be maintained by the incident laser beam, leading to the formation of a large plasma in the blanket gas just above the metal plasma plume.
In fact, this undesirable plasma in the protective gas strongly absorbs the incident laser beam, and this is detrimental to the welding operation. Said mechanism of undesirable plasma generation in the protective gas is known by the name of “inverse Bremsstrahlung”. The incident laser beam may then be strongly disturbed by the blanket gas plasma.
The interaction of the blanket gas plasma with the laser beam may assume various forms, but is usually demonstrated by an effect of absorption and/or diffraction of the incident laser beam, which may substantially reduce the effective laser power density on the surface of the target, causing a decrease in the depth of penetration, or even a coupling failure between the beam and the material, and hence a temporary interruption of the welding process.
The power density threshold above which the plasma appears depends on the ionization potential of the blanket gas used, and is inversely proportional to the square of the laser beam wavelength. Thus, it is very difficult to weld under pure argon with a CO2 laser generator, whereas this operation is feasible with far fewer problems with a YAG laser generator.
In general, in laser welding with a CO2 laser generator, the blanket gas used is helium, which has a high ionization potential and which helps prevent the appearance of the blanket gas plasma, up to laser densities of at least 45 kW.
However, helium has the drawback of being a costly gas, and many laser users prefer to use other gases or gas mixtures which are less expensive than helium but which nevertheless help limit the appearance of the blanket gas plasma, and hence produce welding results similar to those obtained with helium but at lower cost.
Furthermore, L'Air Liquide™, under the trade name LASAL MIX™, sells gas mixtures containing nitrogen and helium or argon and helium, for obtaining substantially the same results as with pure helium, for CO2 laser powers lower than 12 kW, the composition of the mixture concerned being selected or adjusted according to the operating parameters, the laser beam and the material to be welded.
However, these gas mixtures also contain a high proportion of helium, which is not ideal, especially from the economic standpoint.
Accordingly, a problem that arises is to perform a welding operation by laser beam delivered by a CO2 laser generator to generate the welding laser beam, and a protective gas preferably free of helium or containing only a small proportion of helium, that is, much lower than 50% by volume of helium, serving to obtain a weld penetration at least equivalent to that of a laser welding process carried out under the same operating conditions but using helium or a gas containing a high proportion of helium as protective gas, that is, over 50% by volume of helium, and/or which generates little or no plasma in the blanket gas, and preferably regardless of the power (up to about 20 kW) and the focusing conditions of the incident laser beam, and/or which does not lead to a deterioration of the weld quality, in terms of appearance and metallurgical quality (pores, etc.) in the weld bead.